The present invention relates to oil control piston rings and more particularly to three-piece oil control rings of the type including top and bottom split rail rings and a circumferential spacer-expander.
In internal combustion engines, the piston skirt, the cylinder walls and the compression rings are lubricated, cleaned and cooled by lubricant which is splashed or otherwise directed onto the cylinder walls. Oil control rings are necessary to limit the amount of oil which passes into the combustion chamber to thereby reduce emissions, increase engine efficiency and to provide acceptable oil economy. In typical engine designs, lubricant in excess of the amount necessary for lubricating purposes is directed against the cylinder walls. This excess lubricant circulates so as to clean the cylinder walls and also transfers heat to the crankcase or oil sump. The lubricant also increases the effectiveness of the seal between the rings and the cylinder walls and between the rings and the piston.
A wide variety of different oil control rings have been proposed. The prior proposals have included one-piece, two-piece and three-piece oil control rings. The one-piece rings are slotted to provide a return passage for the excess lubricant scraped from the cylinder wall. Two-piece oil control rings include a split rail ring biased against the cylinder wall by a spacer-expander. The spacer-expander has spring elements and integral segmented ring elements. Three-piece oil control rings include top and bottom split rail rings and a spacer-expander.
Examples of the various types of oil control rings which have heretofore been proposed may be found in U.S. Pat. No. 2,635,022, entitled PISTON RING ASSEMBLY, and issued on Apr. 14, 1953, to Shirk; U.S. Pat. No. 2,859,079, entitled TRIPLE SEAL PISTON RING, and issued on Nov. 4, 1958, to Olson; U.S. Pat. No. 3,124,364, entitled PISTON RING ASSEMBLY, and issued on Mar. 10, 1964, to Burns et al; U.S. Pat. No. 3,191,947, entitled PISTON RING, and issued on June 29, 1965, to Hamm; U.S. Pat. No. 3,191,948, entitled PISTON RING, and issued on June 29, 1965, to Hamm; U.S. Pat. No. 3,371,938, entitled PISTON RING, and issued on Mar. 5, 1968, to Hamm et al; U.S. Pat. No. 3,442,519, entitled MONORAIL PISTON RING, and issued on May 6, 1969, to Hamm et al; U.S. Pat. No. 4,053,164, entitled SPACER-EXPANDER FOR A PISTON OIL CONTROL RING, and issued on Oct. 11, 1977, to Saylor; and U.S. Pat. No. 3,741,569, entitled OIL CONTROL PISTON RING, and issued on June 26, 1973, to Mayhew.
The majority of the prior oil control piston rings as exemplified by these aforementioned patents, have attempted to obtain equal contact forces between the cylinder wall and the top and bottom split rails or the top rail and the bottom segmented rail. For example, the aforementioned Mayhew patent discloses a two-piece piston ring for oil control purposes including a plurality of axially, centrally located U-shaped cross-section legs connecting axially top and bottom portions of the ring. The U-shaped cross-section legs which define the spring elements are alternately connected to adjacent legs by circumferential bars at either the top or bottom of the "U". The radial thickness of the bars is controlled so that the spring rates between the top and bottom rail rings are equalized.
The need for equalizing of the forces exerted on the rails and hence on the cylinder wall has generally been accepted commercially even though it has been known that the lower rail of the oil control ring has negligible effect on engine oil economy. It has been demonstrated that the top rail of the oil control ring, which is closer to the high pressure area of the combustion chamber, performs substantially all of the oil control function. Elimination of the lower ring or unequal distribution of the spring forces has not generally been considered desirable since the lower rail structure is needed to effect a counter rotating coupling to the upper rail. The counter rotating coupling insures that the oil control assembly is not twisted and remains stable within the piston ring groove during operation. Prior attempts to design an expander-spacer without the counter rotating coupling have not been successful.